Carbon doping semiconductor devices

ABSTRACT

A method of fabricating a semiconductor device can include forming a III-N semiconductor layer in a reactor and injecting a hydrocarbon precursor into the reactor, thereby carbon doping the III-N semiconductor layer and causing the III-N semiconductor layer to be insulating or semi-insulating. A semiconductor device can include a substrate and a carbon doped insulating or semi-insulating III-N semiconductor layer on the substrate. The carbon doping density in the III-N semiconductor layer is greater than 5×10 18  cm −3  and the dislocation density in the III-N semiconductor layer is less than 2×10 9  cm −2 .

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 14/208,304,filed on Mar. 13, 2014, which claims priority to U.S. ProvisionalApplication No. 61/791,395, filed on Mar. 15, 2013. The disclosure ofthe prior applications are considered part of and are incorporated byreference in the disclosure of this application.

TECHNICAL FIELD

This disclosure relates generally to fabricating semiconductor devices,and in particular to carbon doping semiconductor devices.

BACKGROUND

Many transistors used in power electronic applications have beenfabricated with silicon (Si) semiconductor materials. Common transistordevices for power applications include Si CoolMOS, Si Power MOSFETs, andSi Insulated Gate Bipolar Transistors (IGBTs). While Si power devicesare inexpensive, they suffer from a number of disadvantages, includingrelatively low switching speeds and high levels of electrical noise.More recently, silicon carbide (SiC) power devices have been considereddue to their superior properties. III-Nitride or III-N semiconductordevices, such as gallium nitride (GaN) based devices, are now emergingas attractive candidates to carry large currents, support high voltages,and to provide very low on-resistance and fast switching times.

SUMMARY

A method of fabricating a semiconductor device can include forming aIII-N semiconductor layer in a reactor and, while forming the III-Nsemiconductor layer, injecting a hydrocarbon precursor into the reactor,thereby carbon doping the III-N semiconductor layer and causing theIII-N semiconductor layer to be insulating or semi-insulating.

A semiconductor device can include a substrate and a carbon dopedinsulating or semi-insulating III-N semiconductor layer on thesubstrate. The carbon doping density in the III-N semiconductor layer isgreater than 5×10¹⁸ or 1×10¹⁹ cm⁻³, and a dislocation density in theIII-N semiconductor layer is less than 2×10⁹ cm⁻².

Methods and devices described herein can each include one or more of thefollowing features. Injecting the hydrocarbon precursor can compriseinjecting a hydrocarbon precursor having a chemical formula(C_(x)H_(y)), where x and y are integers greater than or equal to 1.Forming the III-N semiconductor layer on the substrate can compriseforming the III-N semiconductor layer as a III-N buffer layer over aIII-N nucleation layer over a silicon substrate. Methods can compriseforming a III-N channel layer over the III-N buffer layer and forming aIII-N barrier layer over the III-N channel layer, thereby forming atwo-dimensional electron gas (2DEG) active channel adjacent to aninterface between the channel layer and the barrier layer. Forming theIII-N semiconductor layer as a III-N buffer layer can comprise formingthe III-N buffer layer under a plurality of growth conditions, andforming the III-N channel layer can comprise forming the III-N channellayer under the same or substantially the same growth conditions. Theplurality of growth conditions can comprise a surface temperature and areactor pressure. The plurality of growth conditions can furthercomprise a ratio of group-III precursor flow rate to group-V precursorflow rate. Forming the III-N semiconductor layer on the substrate cancomprise forming the III-N semiconductor layer by metal organic chemicalvapor deposition (MOCVD). The barrier layer can comprise AlGaN, thechannel layer can comprise undoped or unintentionally doped (UID) GaN,and the buffer layer can comprise AlGaN or GaN or both.

Forming the III-N semiconductor layer can comprise injecting a group-IIIprecursor into the reactor at a group-III precursor molar flow rate, andinjecting the hydrocarbon precursor into the reactor can compriseinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate, wherein the hydrocarbon precursor molar flowrate is at least 0.02 times the group-III precursor molar flow rate.Forming the III-N semiconductor layer can comprise injecting a group-IIIprecursor into the reactor at a group-III precursor molar flow rate, andinjecting the hydrocarbon precursor into the reactor can compriseinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate, wherein the hydrocarbon precursor molar flowrate is greater than the group-III precursor molar flow rate. Thehydrocarbon precursor can comprise propane or methane or both. Methodscan further comprise adding a gate terminal, a drain terminal, and asource terminal to the semiconductor device, thereby forming a III-Nhigh electron mobility transistor (HEMT). Methods can further compriseadding an anode terminal and a cathode terminal to the semiconductordevice, thereby forming a III-N diode. Causing the III-N semiconductorlayer to be insulating or semi-insulating can comprise causing the III-Nsemiconductor layer to have a resistivity of at least 1×10⁷ ohm-cm.Carbon doping the III-N semiconductor layer can result in the III-Nsemiconductor layer having a carbon concentration greater than ×10¹⁸cm⁻³.

The III-N semiconductor layer can have a first side distal from thesubstrate and a second side between the first side and the substrate,wherein the dislocation density in the III-N semiconductor layer is adislocation density adjacent to the first side of the III-Nsemiconductor layer. The III-N semiconductor layer can comprise a III-Nbuffer layer over a III-N nucleation layer, wherein the substrate is asilicon substrate. Devices can further comprise a III-N channel layerover the III-N buffer layer and a III-N barrier layer over the III-Nchannel layer, thereby forming a two-dimensional electron gas (2DEG)active channel adjacent to an interface between the channel layer andthe barrier layer. The barrier layer can comprise AlGaN, the channellayer can comprise undoped or unintentionally doped (UID) GaN, and thebuffer layer can comprise AlGaN or GaN or both. The substrate can be aforeign substrate. Devices can further comprise a gate terminal, a drainterminal, and a source terminal, wherein the semiconductor device is aIII-N high electron mobility transistor (HEMT). Devices can furthercomprise an anode terminal and a cathode terminal, wherein thesemiconductor device is a III-N diode. The carbon doping density in theIII-N semiconductor layer can be less than 5×10²¹ cm⁻³.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. An insulating or semi-insulating carbon dopedIII-N layer can be formed with a level of carbon doping from a widerange of concentrations (1e16-1e22 cm⁻³) with fewer restrictions on oneor more growth parameters of the layer compared to conventionaltechnology. Insulating or semi-insulating layers can be formed with lowdislocation densities and smooth surfaces grown on foreign substrates,e.g., Si or SiC substrates. Injecting a halogen free precursor (e.g., ahydrocarbon precursor) during metalorganic chemical vapor deposition(MOCVD) can reduce or eliminate interactions of halogen containingmolecules with the metalorganic precursors, thereby avoiding theinfluence of CX₄ (X=halogen) precursors on an alloy composition (i.e.,the ratio of Al to Ga in AlGaN) during MOCVD growth of carbon dopedAlGaN.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are cross-sectional views of an example III-Nsemiconductor device.

FIG. 2 is a flow diagram of an example method for fabricating a III-Nsemiconductor device including a carbon doped layer.

FIG. 3 is a block diagram of a system for fabricating a III-Nsemiconductor device with at least one layer that is carbon doped.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1A is a cross-sectional view of an example III-Nitride (i.e.,III-N) semiconductor device 100. For example, the device can be atransistor, e.g., a III-N high electron mobility transistor (HEMT), byadding source 114, drain 116, and gate 118 terminals to the device, asillustrated in FIG. 1B. In another example, the device can be a diode byadding anode and cathode terminals to the device (not shown).

The device includes a substrate 102. The substrate can be, e.g.,silicon, SiC, aluminum nitride (AlN), GaN, sapphire (Al₂O₃), or anyother suitable growth substrate for the growth of III-N materials.Because large native substrates (i.e., substrates formed of III-Nmaterials) are currently unavailable and tend to be very expensive, thedevice is typically formed on a foreign substrate (i.e., a substrateformed of a material that is not a III-N material), such as silicon,silicon carbide, or sapphire. The device includes a nucleation layer 104on the substrate. The nucleation layer can be a III-N nucleation layerand can include, e.g., AlN.

The device includes a buffer layer 106. The buffer layer can be a III-Nbuffer layer and can include, e.g., C-doped AlGaN or GaN or both. Thebuffer layer can include several different layers, e.g., with somelayers closer to the substrate having a higher concentration of Al andsome layers further from the substrate having a lower concentration ofAl. The buffer layer can be made insulating or semi-insulating by carbondoping the buffer layer. This can be useful, e.g., to prevent subsurfaceleakage or premature breakdown.

The device includes a III-N channel layer 108 and a III-N barrier layer110, where the compositions of the channel layer and the barrier layerare selected to induce a two-dimensional electron gas (2DEG) 112 activechannel adjacent to an interface between the channel layer and thebarrier layer. For example, the channel layer can include undoped orunintentionally doped (UID) GaN and the barrier layer can include AlGaN.

The terms III-Nitride or III-N materials, layers, devices, andstructures can refer to a material, device, or structure comprised of acompound semiconductor material according to the stoichiometric formulaB_(w)Al_(x)In_(y)Ga_(z)N, where w+x+y+z is about 1, and w, x, y, and zare each greater than or equal to zero and less than or equal to 1. In aIII-Nitride or III-N device, the conductive channel can be partially orentirely contained within a III-N material layer.

The layers of the device can be formed by molecular beam epitaxy (MBE)or metalorganic chemical vapor deposition (MOCVD) in a reactor or othertechniques. One way to achieve carbon doping in a III-N layer formed byMOCVD with NH₃ as the nitrogen precursor is to adjust the layer growthconditions so that carbon from metalorganic precursors (e.g., TMGa orTMAl or both) is incorporated into the layers. For example, some growthconditions that favor the incorporation of carbon include: low reactorpressure, low NH₃ partial pressure, low deposition temperatures, andhigh growth rates.

When these growth conditions are implemented for carbon doping at levelssufficient to cause a layer to be insulating or semi-insulating forcertain applications, the growth conditions are limited for calibrationwith respect to other features of the layer, e.g., threading dislocationdensity and surface roughness of the layer. For example, consider alayer formed on a foreign (i.e., non-III-N) substrate, e.g., silicon(Si), silicon carbide (SiC), or sapphire (Al₂O₃).

Such a layer can be formed under growth conditions including one or moreof lower reactor pressure, lower NH₃ partial pressure, lower depositiontemperatures, and higher growth rates, but these growth conditions canalso result in higher dislocation densities and higher levels of pointdefects in the layer. Increasing carbon doping levels to greater thanabout 5×10¹⁸ cm⁻³ using these methods can additionally result in surfaceroughening or poor surface morphology or both.

Another way to achieve carbon doping in a layer is to inject ahydrocarbon precursor into the reactor during growth of the layer.Hydrocarbon precursors include molecules of the chemical composition(C_(x)H_(y)), where x and y are integers greater than or equal to 1.Examples of hydrocarbons include propane (C₃H₈), methane (CH₄), andC₂H₂.

This way of achieving carbon doping can result in the layer havingcarbon doping in excess of 1×10¹⁸, 5×10¹⁸, 1×10¹⁹, or 3×10¹⁹ cm⁻³ whilesimultaneously having a dislocation density less than 2×10⁹ cm⁻², forexample about 1×10⁹ cm⁻² or less or about 8×10⁸ cm⁻² or less. The carbondoping density in the III-N semiconductor layer can be between 1×10¹⁹cm⁻³ and 5×10²¹ cm⁻³. In some implementations, the nucleation layer isbetween 20-50 nm thick, the buffer layer is between 1-10 microns thick(e.g., about 5 microns), the channel layer is about 200-1000 nm thick(typically about 400 nm), and the barrier layer is between 100-400 nmthick (e.g., about 250 nm).

FIG. 2 is a flow diagram of an example method 200 for fabricating aIII-N semiconductor device including a carbon doped layer. For purposesof illustration, the method will be described with reference to theexample device 100 of FIG. 1, but the method can be used to fabricateother devices and to carbon dope other types of layers in other devices.

A nucleation layer is formed on a silicon substrate (202). For example,the silicon substrate can be placed into a reactor such as an MOCVDreactor, and the nucleation layer can be deposited, e.g., as a layer ofAlN within the reactor.

A buffer layer is formed on the nucleation layer (204). For example, thebuffer layer can be deposited, e.g., as a layer of AlGaN or GaN or both.In some implementations, the buffer layer comprises more than one layer.Layers of AlGaN are deposited, with a decreasing amount of Al in eachsuccessive layer. Eventually, one or more layers of GaN can bedeposited.

While the buffer layer is formed, a hydrocarbon precursor is injectedinto the reactor (206). For example, the hydrocarbon precursor can beinjected into the reactor simultaneously or alternately while injectinggroup III and/or group V precursors into the reactor.

A channel layer is formed on the buffer layer (208). For example, thechannel layer can be deposited, e.g., as a layer of undoped orunintentionally doped (UID) GaN. In some implementations, the channellayer is formed under the same or substantially the same growthconditions as the buffer layer. Where the buffer layer includes a toplevel layer of GaN, the channel layer can be deposited by ceasing toinject the hydrocarbon precursor and continuing to deposit GaN withoutaltering any other growth conditions in the reactor. That is, thereactor pressure and/or temperature and/or the total gas molar flow rateinto the reactor and/or the ratio of group V precursor molar flow rateto group III precursor molar flow rate can be the same for the channellayer and for the portion of the buffer layer that is directly adjacentto the channel layer, with a hydrocarbon precursor injected into thereactor during growth of the portion of the buffer layer that isdirectly adjacent to the channel layer but not during growth of thechannel layer.

A barrier layer is formed on the channel layer (210). For example, thebarrier layer can be deposited, e.g., as a layer of AlGaN. Atwo-dimensional electron gas (2DEG) active channel is induced adjacentto an interface between the channel layer and the barrier layer. Thebarrier layer can have a larger bandgap than the channel layer, whichcan in turn at least partially cause the 2DEG to be induced in thechannel layer. To form a transistor, source, gate, and drain terminalsare then formed on the III-N material layer structure (212).

FIG. 3 is a block diagram of a system 300 for fabricating a III-Nsemiconductor device with at least one layer that is carbon doped. Thesystem can be used, for example, to perform the method of FIG. 2 tofabricate the device of FIGS. 1A and 1B.

The system includes a reactor 302, e.g., an MOCVD reactor. A substrate304 is placed into the reactor and a III-N layer 306 is formed on thesubstrate. A reactor control system 308 controls the formation of thelayer 306 by adjusting one or more growth conditions. The reactorcontrol system can control the injection of one or more materials intothe reactor, including carrier gases 316 (e.g., an inert carrier gassuch as H₂ or N₂ or both), group-V precursor gases 318 (e.g., NH₃),group-III precursor gases 320 (e.g., TMGa or TMAl or both), andhydrocarbon precursor gases 322 (e.g., one or more of C₃H₈, CH₄, andC₂H₂).

The reactor control system can be implemented, e.g., as a system of oneor more computers that receives input from an operator and providesoutput control signals, e.g., to the reactor and storage modules for thegases. The reactor control system can include a pressure control module310 (e.g., to control the pressure in the reactor), a depositiontemperature control module 312 (e.g., to control the surface temperatureof a layer being formed), a growth rate module 314, and other modules,for example. The growth rate module 314 may control the growth rateindirectly by controlling variables which affect the growth rate, suchas reactor pressure, surface temperature, and flow rates of the variousprecursors and carrier gases.

In some implementations, the reactor control system is configured toform the III-N semiconductor layer by injecting a group-III precursorinto the reactor at a group-III precursor molar flow rate and byinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate. The amount of carbon doping in the layer canbe at least partially controlled by varying the ratio between thehydrocarbon precursor molar rate and the group-III precursor molar flowrate.

It has been found that for some hydrocarbon precursors for carbon dopingof III-N materials during MOCVD growth of the III-N materials, inparticular propane (C₃H₈), the dopant incorporation efficiency is muchlower than the incorporation efficiency of other dopants typicallyintroduced during MOCVD growth of III-N materials. For example, for adopant such as silicon, where silane or disilane is used as the siliconprecursor, when the ratio of the silicon precursor molar flow rate tothe group-III precursor molar flow rate is about 1/1000 (and in somecases even lower), the silicon doping level in the III-N material isapproximately equal to the saturation limit of the dopant in the III-Nmaterial, which may be around 1×10²¹ cm⁻³. Increasing the siliconprecursor molar flow rate relative to the group-III precursor molar flowrate to a higher value does not substantially increase the silicondoping level, and typically results in a poorer structural quality ofthe resulting III-N layer, for example leading to higher dislocation andpoint defect densities, as well as poor surface morphology. However, forcarbon doping of III-N materials during MOCVD growth using propane asthe carbon precursor, when the growth is performed under reactorconditions that correspond to low carbon doping levels (e.g., less than1×10¹⁷ cm⁻³) in the absence of the propane precursor, adding propane ata molar flow rate of about 1/1000 that of the group-III precursor molarflow rate does not substantially increase the carbon doping in the III-Nmaterial, and typically still yields a carbon doping level which is lessthan 1×10¹⁷ cm⁻³.

In some systems, and in particular when propane (C₃H₈) is utilized asthe hydrocarbon precursor, a hydrocarbon precursor molar flow rate whichis about or at least 0.02 times the group-III precursor molar flow ratemay be needed in order for the carbon doping level in the layer to bebetween about 1×10¹⁷ and 1×10¹⁹ cm⁻³, or to be in excess of 1×10¹⁷ cm⁻³.In some systems, when the hydrocarbon precursor molar flow rate is aboutor at least 0.2 times the group-III precursor molar flow rate, thecarbon doping level in the layer can be about or in excess of 1×10¹⁸cm⁻³, or between about 1×10¹⁸ and 1×10²⁰ cm⁻³. In some systems, when thehydrocarbon precursor molar flow rate is substantially greater than thegroup-III precursor molar flow rate, e.g., 2 times or 20 times thegroup-III precursor molar flow rate, the carbon doping level in thelayer can be about or in excess of 1×10¹⁹ or 1×10²⁰ cm⁻³. Theresistivity of a carbon doped layer formed with propane precursors canbe greater than 1×10⁷ ohm-cm for carbon doping levels of about 1×10¹⁹cm⁻³ and greater than 1×10⁸ ohm-cm for carbon doping levels of about1×10²⁰ cm⁻³.

In some implementations, the reactor control system is configured toform at least one layer (e.g., the UID GaN channel layer) at a surfacetemperature of 1077 C and a pressure of 200 mBarr. The reactor controlsystem flows the nitrogen precursor, e.g., ammonia (NH₃), into thereactor at a rate of 0.54 mol/min, flows tri-methyl gallium (TMGa) intothe reactor at a rate of 0.65 milli-mol/min, and controls the total gasflow into the reactor to at or about 80 liters per minute. The reactorcontrol system can maintain the total gas flow at a substantiallyconstant rate by increasing or decreasing the carrier gas flow tocompensate for increases or decreases in other flows. This results incarbon doping of about 5×10¹⁶ cm⁻³ or lower in this layer.

The reactor control system can form the carbon doped layer under thesame or substantially the same growth conditions by flowing thehydrocarbon precursor into the reactor. For example, for the carbondoped layer, if the surface temperature is maintained at 1077 C, thepressure is maintained at 200 mBarr, the ammonia flow rate is maintainedat 0.54 mol/min, the TMGa flow rate is maintained at 0.65 milli-mol/min,and the rate of total gas flow into the reactor is maintained at about80 liters per minute, by flowing a hydrocarbon precursor into thereactor, carbon doping levels of greater than 1×10¹⁸ cm⁻³, greater than1×10¹⁹ cm⁻³, or greater than 1×10²⁰ cm⁻³ can be achieved. At the sametime, if the carbon doped III-N layer is formed on a foreign substratesuch as silicon, the dislocation density of the upper portion of thecarbon doped III-N layer (i.e., the portion adjacent to the surface ofthe carbon doped III-N layer which is furthest from the substrate) canbe maintained at a level smaller than 2×10⁹ cm⁻², and typically evensmaller than 1×10⁹ cm⁻².

By way of comparison, if the hydrocarbon precursor is not flowed intothe reactor during growth of the carbon doped layer, the reactor controlsystem can adjust one or more or all of the growth parameters toincorporate enough carbon to cause the carbon doped layer to becomeinsulating to a specified degree. For example, the reactor controlsystem can reduce the pressure to 50 mBarr, reduce the temperature to1020 C, reduce the NH₃ flow rate to 0.045 mol/min, maintain the totalgas flow at about 80 liters per minute, and maintain the flow ofgroup-III precursor gases.

These adjustments to the growth conditions can result in carbon dopingof up to about 5×10¹⁸ cm⁻³. The dislocation density at the upper surfaceof the layer when the layers are grown under these conditions can begreater than 2×10⁹ cm⁻², and is typically between 5×10⁹ and 6×10⁹ cm⁻².Further adjusting the reactor conditions to further increase the carbonconcentration in these layers can cause substantial degradation in thesurface morphology of the material structure, and typically also resultsin even higher dislocation densities.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the techniques and devices describedherein. For example, the processes described herein for forming carbondoped III-N layers can be used in the fabrication of other devices thatrequire insulating or semi-insulating layers, e.g., photovoltaic cells,lasers, and LEDs. Accordingly, other implementations are within thescope of the following claims.

What is claimed is:
 1. A method of fabricating a semiconductor materialstructure, the method comprising: forming a III-N semiconductor layer ona substrate in a reactor, the forming of the III-N semiconductor layercomprising simultaneously injecting into the reactor a first precursor,a second precursor different from the first precursor, and a group-Vprecursor, wherein the first precursor is a group-III precursor and thesecond precursor is a hydrocarbon precursor, and the injecting of thehydrocarbon precursor causes the III-N semiconductor layer to be carbondoped.
 2. The method of claim 1, wherein the hydrocarbon precursorcomprises molecules having a chemical formula (C_(x)H_(y)), where x andy are integers greater than or equal to 1, and the group-III precursorcomprises a metalorganic precursor.
 3. The method of claim 1, whereinforming the III-N semiconductor layer on the substrate comprises formingthe III-N semiconductor layer as a III-N buffer layer over a III-Nnucleation layer over a silicon substrate.
 4. The method of claim 3,further comprising forming a III-N channel layer over the III-N bufferlayer and forming a III-N barrier layer over the III-N channel layer,thereby forming a two-dimensional electron gas (2DEG) active channeladjacent to an interface between the channel layer and the barrierlayer.
 5. The method of claim 4, wherein the barrier layer comprisesAlGaN, the channel layer comprises undoped or unintentionally doped(UID) GaN, and the buffer layer comprises AlGaN or GaN or both.
 6. Themethod of claim 1, wherein the hydrocarbon precursor comprises propaneor methane or both.
 7. The method of claim 1, wherein causing the III-Nsemiconductor layer to be carbon doped results in the III-Nsemiconductor layer having a carbon concentration greater than ×10¹⁸cm³.
 8. The method of claim 1, wherein forming the III-N semiconductorlayer on the substrate comprises forming the III-N semiconductor layerby metal organic chemical vapor deposition (MOCVD).
 9. A method offabricating a semiconductor material structure, the method comprising:forming a III-N semiconductor layer on a substrate in a reactor, theforming of the III-N semiconductor layer comprising simultaneouslyinjecting into the reactor a group-III precursor selected from the groupconsisting of TMGa and TMAl, a group-V precursor, and a hydrocarbonprecursor comprising molecules having a chemical formula (C_(x)H_(y)),where x and y are integers greater than or equal to 1; wherein theinjecting of the hydrocarbon precursor causes the III-N semiconductorlayer to be carbon doped.
 10. The method of claim 9, wherein theinjecting of the group-III precursor into the reactor comprisesinjecting the group-III precursor at a group-III precursor molar flowrate, and the injecting of the hydrocarbon precursor into the reactorcomprises injecting the hydrocarbon precursor at a hydrocarbon precursormolar flow rate different from the group-III precursor molar flow rate.11. The method of claim 10, wherein the hydrocarbon precursor molar flowrate is at least 0.02 times the group-III precursor molar flow rate. 12.The method of claim 11, wherein the hydrocarbon precursor molar flowrate is greater than the group-III precursor molar flow rate.
 13. Themethod of claim 9, wherein the hydrocarbon precursor comprises propaneor methane or both.
 14. The method of claim 9, wherein forming the III-Nsemiconductor layer on the substrate comprises forming the III-Nsemiconductor layer by metal organic chemical vapor deposition (MOCVD).15. A method of fabricating a semiconductor material structure, themethod comprising: forming a first III-N semiconductor layer in areactor, the first III-N semiconductor layer being carbon doped; andforming a second III-N semiconductor layer on the first III-Nsemiconductor layer in the reactor; wherein the forming of the firstIII-N semiconductor layer comprises injecting into the reactor agroup-III precursor, a group-V precursor, and a hydrocarbon precursor,and the forming of the second III-N semiconductor layer comprisesinjecting into the reactor the group-III precursor and the group-Vprecursor but not the hydrocarbon precursor.
 16. The method of claim 15,wherein the second III-N semiconductor layer is undoped orunintentionally doped.
 17. The method of claim 15, wherein the group-IIIprecursor comprises TMGa or TMAl, the group-V precursor comprisesammonia, and the hydrocarbon precursor comprises molecules having achemical formula (C_(x)H_(y)), where x and y are integers greater thanor equal to
 1. 18. The method of claim 15, wherein the carbonconcentration in the first III-N semiconductor layer is in excess of1×10¹⁷ cm⁻³.
 19. The method of claim 15, wherein the hydrocarbonprecursor comprises propane or methane or both.
 20. The method of claim15, wherein forming the III-N semiconductor layer on the substratecomprises forming the III-N semiconductor layer by metal organicchemical vapor deposition (MOCVD).